test/zdtm/Makefile.inc: don't regen all deps

As it was pointed out by our esteemed maintainer (let his light shine),
after my recent changes to test/zdtm Makefiles all dependencies are
regenerated even if we only need to build a single test (for example,
cd test/zdtm/static && make env00).

This was caused by "-include $(DEP)" statement. Make sees that these files
are need to be included, but are missing, and since it knows how to generate
them it goes on to do so.

The solution is to use $(wildcard) function which returns the list of
_existing_ files, and so include will only receive the files that exist.
Reported-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

test/zdtm/Makefile.inc

@@ -89,7 +89,7 @@ dep: $(DEP)
 no-deps-targets	:= clean cleandep cleanout realclean groups.cleanout
 
 ifeq ($(filter $(no-deps-targets), $(MAKECMDGOALS)),)
--include $(DEP)
+-include $(wildcard $(DEP))
 endif
 
 .SECONDARY: